1. Field of the Invention
The present invention relates to The present invention relates to audio systems and, more particularly, to an audio system which communicatively interconnects an audio apparatus for reproducing and outputting audio signals and an electronic device for outputting voice guidance via a speaker connected to the audio apparatus, and which is designed to facilitate listening of a voice signal having high priority (hereinafter referred to as “a high-priority voice signal”) in outputting it as the guidance voice; to an audio apparatus therein; and to a method for performing audio signal output processing.
The “high-priority voice signal” as described herein indicates important voice guidance information which needs to be recognized by a user. For example, in the case of a vehicle audio system with a vehicle audio apparatus and a navigation apparatus integrated therewith, the high-priority voice signal can represent voice guidance concerning the guidance at an intersection (e.g. a message to turn right after 300 m), which is given by the navigation apparatus to a driver.
2. Description of the Related Art
In recent years, vehicles have been developed to have various kinds of electronic devices installed therein, such as a CD player, a DVD player, an AM/FM tuner, a cassette tape deck, a navigation apparatus, a telephone set, and the like. Most of these electronic devices are stand-alone and are interconnected to one another via common signal lines, such as a bus, to constitute an audio/navigation system. In such a system, a normal method of outputting voice guidance for navigation involves, for example, (i) outputting voice guidance from the respective speakers which are provided in the audio apparatus and in the navigation apparatus. Another prior method for outputting voice guidance involves, for example, (ii) mixing an audio signal and a voice guidance signal for navigation in the audio device to output the mixed result.
Although most of the simple systems employ the method (i), this method disadvantageously renders it difficult for the user (typically, a driver) to listen to the voice guidance when the audio signal and the voice guidance signal for navigation are simultaneously produced. For this reason, in recent years, the method (ii) has been often employed and is designed to permit the user to easily listen to the high-priority voice signal including the voice guidance for navigation.
In this method (that is, the method including mixing and outputting the voice guidance signal for navigation and the audio signal), as shown in one example of the audio signal output processing of FIG. 6, for example, even when the voice guidance signal (high-priority voice signal) GS is received with the audio signal AS being received, a mixing operation (AS+GS) of both signals is carried out in the audio apparatus and a filtering operation is appropriately executed, so that the guidance voice signal GS can be selectively produced. That is, only the high-priority voice signal GS can be listened to.
In recent normal audio systems, most audio signals are recorded in digital form in a recording medium such as a CD, a DVD, an MD, or the like, regardless of whether the audio system is an in-car product or not. The signals in digital form are subjected to digital signal processing by means of a digital signal processor (DSP). In a final stage, they are often subjected to digital/analog (D/A) conversion. In a system aiming for higher audio quality, adjustment of the signals is given by the digital signal processing with the DSP such that each speaker (a woofer, a tweeter, or the like) has its appropriate characteristics, and then the digital signals are converted into analog signals to be produced.
In these systems, the audio signals are finally produced in analog form. Since a main audio signal source has hitherto been the AM/FM tuner, the cassette tape deck, or the like, most of the audio systems which are integrated with electronic devices such as the navigation apparatus, the telephone set, or the like, in most cases perform the mixing operation in analog form in the audio signal output processing. For example, as shown in one example of FIG. 7(a), in the audio processor, an audio signal from the DVD player or the like in digital form is subjected to digital signal processing by the DSP 51, and then is converted into an analog signal through the D/A converter 52. Thereafter, this analog audio signal and the voice guidance signal from the navigation apparatus or the like in analog form are mixed by the mixing processor 53.
In this case, depending on the output characteristics of the speaker, there may be a problem. For example, in a tweeter, a worse response of the speaker is induced by the input of low frequency components. In contrast, in a woofer, which is a speaker having the characteristics of producing sounds only at the low audio frequencies, a worse response of the speaker is not caused, but output sound pressures at middle and high frequencies become low, thus resulting in difficulty in hearing these sounds. Taking into consideration the foregoing, in the mixing operation of the guidance voice, measures are required to execute the mixing operation for channels with wide reproduction bands, such as channels in full ranges. In cases where the recent vehicle audio system aiming for high audio quality has the navigation apparatus integrated therewith, strict execution of the output processing needs to distribute and output the signals via filters for the respective channels, depending on the characteristics of the speakers connected to the system. In addition, it is necessary to adjust the characteristics of each filter in accordance with the characteristics of the combined signals.
On the other hand, in recent years, audio signals often have been processed in digital form. Some systems including the DSP execute the mixing operation by the DSP. For example, as shown in one example of FIG. 7(b), in the audio processor, an audio signal from the DVD player or the like in digital form is provided to a mixing processor 55 of a DSP 54, together with a voice guidance signal from the navigation apparatus or the like in digital form, and then is subjected to the digital mixing. Thereafter, the mixed signal is converted into an analog signal through a D/A converter 56, and amplified through an amplifier 57 to be produced therefrom.
As a known technique associated with the foregoing prior art, for example, a navigation system includes an on-vehicle navigation apparatus which produces voice guidance via a speaker connected to a vehicle audio device, wherein voice guidance is produced during intervals between pieces of music reproduced by the vehicle audio device, as disclosed in Japanese Unexamined Patent Publication No. 2001-116581, for example. As another technique, a navigation system adjusts a mixing condition of the volume of an audio device and of the volume of the voice guidance from the navigation apparatus in accordance with the importance of the voice guidance generated from the navigation apparatus, as disclosed in Japanese Unexamined Patent Publication No. 2002-116045, for example.
As mentioned above, prior vehicle audio systems employ the method including mixing the voice guidance signal and the audio signal to produce the mixed result in order to enable easy listening of the high-priority voice signal (voice guidance signal) such as voice guidance for navigation. In this case, however, since there are various kinds of media such as a CD, a DVD, an MD, and the like as described above, the audio signal to be mixed is often switched from one sampling frequency (sample rate) to another, depending on the type of medium. For example, in the case of the CD or the MD, the audio signal is switched to a sample rate of 44.1 kHz, while in the case of the DVD, the audio signal is switched to 48 kHz, 96 kHz, or 192 kHz. Moreover, audio data (music) encoded by an audio compression technique, such as MP3 (MPEG1, audio layer 3), can be recorded and edited in a disc by the user, such as a CD-R, or the like, and this disk has been often reproduced via an audio device. In the case of such a disk, e.g. the CD-R, the sampling frequency varies with every piece of music (i.e. track) at frequent intervals.
When the sample rate is switched, if the data before switching of the sample rate is reproduced at a new sample rate taken after switching, a tone goes high or low, resulting in unnatural sounds. To prevent this phenomenon, it is necessary to switch a filter coefficient (that is, the selection characteristics of an audio output frequency band) according to the sample rate. More concretely, under control of a microcomputer or the like, after data inside the DSP is cleared, the sample rate is switched to the other and a level of the audio signal is adjusted to restart reproduction.
However, since the output timing of voice guidance for the navigation or the like is provided at ii regular intervals, in fact, it is impossible that this output timing is completely off for the duration of the foregoing sample-rate switching operation. As a result, if the timing of the sample-rate switching operation overlaps with the timing of the voice guidance for navigation, the output of the voice guidance signal GS is interrupted, that is, so-called “sound interruption” occurs, during the sample-rate switching operation, as shown in one example of the audio signal output processing in FIG. 8, thus disadvantageously rendering it difficult for the user to listen to the navigation voice guidance or the like at least in part.